UI Tests: Update to the new Galata setup

[jtpio]

Problem

The new galata setup now lives in the core JupyterLab repo: https://github.com/jupyterlab/jupyterlab/tree/master/galata

We should update to it, as it comes with many improvements.

Proposed Solution

Update the ui-tests setup here: https://github.com/voila-dashboards/voila/tree/master/ui-tests

Following the instructions here: https://github.com/jupyterlab/jupyterlab/tree/master/galata

Additional context

Galata was moved to the core lab repo in jupyterlab/jupyterlab#10796

[jtpio]

Looks like the current UI Tests check is still a bit flaky. So doing the update might help with that.